Quantizer matrices are encoded in zigzag scan order in the bitstream,
but they are stored in raster scan order when they are parsed. However,
default matrices were also prepared in zigzag scan order, hence the
mismatch. i.e. the matrices were presented either in raster scan order
if they are explicitly present in the bitstream, or they were presented
in zigzag scan order if the default definitions were to be used instead.

One way to solve this problem is to always expose the quantization
matrices in zigzag scan order, since this is the role of the parser to
not build up stories from the source bitstream and just present what
is in there.

Utility functions will be provided to convert quantization matrices in
either scan order.